The twisted nematic (TN) liquid crystal element or device using a low-molecular-weight nematic liquid crystal in combination with polarizer film has been used in a diversity of fields. In parallel with the recent popularity of TN liquid crystal display elements or devices, there is a mounting interest in the possible exploitation of other kinds of liquid crystals such as high-molecular-weight liquid crystals in display elements or devices. However, high-molecular-weight liquid crystals, which are comparatively large in molecular size, are not so mobile that when the electric field is turned on or off for display control, it takes a long time for the orientation or disorientation of the polymer backbone chain to be consummated. Therefore, it has been considered impossible to implement an electric field-controllable display device using a high-molecular-weight liquid crystal.
Meanwhile, as liquid crystals other than nematics, smectic and cholesteric liquid crystals are known. The cholesteric liquid crystal, which has a periodic structure (helicoidal molecular orientation), selectively reflects a given wavelength component of light so that unlike the TN liquid crystal display, the cholesteric LC display element or device does not require a color filter or a polarizer. Therefore, display devices employing cholesteric liquid crystals feature sharp and bright display colors and can find application as a variety of optical media. However, because of their principle of display, the display colors of cholesteric liquid crystals can hardly be controlled by an electric or magnetic field, with the result that it has been generally believed that those liquid crystals cannot be effectively implemented as color display elements or devices.
Referring to such cholesteric liquid crystals, Japanese Patent Publication No. 23471/1995 (JP-B-7-23471) discloses a lyotropic liquid crystal composition forming a liquid crystal phase upon mixing with a solvent, which comprises a solution of a hydroxyethylcellulose in water, an organic solvent, or a mixture of such solvents.
Japanese Patent Application Laid-open No. 220453/1994 (JP-A-6-220453) discloses a liquid crystal composition comprising a lyotropic cholesteric liquid crystal composed of a linear homopolysaccharide derivative, typically hydroxypropylcellulose, and water supplemented with 0.05 to 5 weight % of a water-soluble electrolyte and a laminate structure fabricated by layering the composition on a substrate which is at least partially transparent. This literature states that the above liquid crystal composition undergoes a transition from a liquid crystal phase to a random cohesive or aggregate phase to develop opacity by scattering visible light, that addition of a water-soluble electrolyte to the composition depresses the opacification temperature of the liquid crystal composition to room temperature or a temperature close to room temperature, and that an automatic switching from the transparent liquid crystal phase to the opaque scattered phase or vice versa can be achieved with direct solar radiation energy. However, none of the literature suggest the feasibility of color display by means of an electric field.